1. Field of the Invention
The present invention relates to a technique for holding a circuit board. In particular, the invention relates to a technique for holding a flexible circuit board when mounting electronic components on the flexible circuit board. More particularly, the invention relates to a substrate holder for holding a flexible circuit board by adhesion.
2. Description of the Background Art
In recent years, sheet-like flexible printed circuit boards (hereinafter referred to as “FPCs”) have been used in small electronic devices such as mobile phones, PDAs (Personal Data Assistants), and notebook-type computers. An FPC comprises a resin sheet having formed thereon various wiring patterns on which a variety of electronic components (e.g., ICs, capacitors, resistors, coils, and/or connectors) are mounted. By using an FPC in an electronic device, it is possible to flexibly dispose a circuit board in the electronic device and to downsize the electronic device.
The FPC, however, has a disadvantage in that it is difficult to handle by itself without any support, due to its flexibility. In particular, when mounting electronic components on an FPC, various processes are performed on a surface of the FPC, and thus it is essential to securely hold the FPC. In order to treat FPCs in the same manner as conventional plate-like circuit boards (e.g., circuit boards formed of glass epoxy resin or the like), conventionally, an FPC is fixed onto a plate having high stiffness with clamps.
Japanese Patent Laid-Open Publication No. 2002-232197 discloses a technique in which an adhesive layer is provided on a base plate of a pallet to allow an FPC to adhere thereto, whereby the FPC along with the base plate can be treated in the same manner as a conventional circuit board. In addition, Japanese Patent No. 3435157 describes a jig having adhesive layers formed thereon in a plurality of locations so that a plurality of FPCs are held to the adhesive layers.
When mounting electronic components on an FPC being adhered onto a base plate, the shape and/or tackiness of an adhesive region to be provided between the FPC and the base plate becomes important. For example, when mounting electronic components on an FPC, normally, various processes are performed on the FPC being adhered onto a base plate, which include screen-printing of solder paste, installation of electronic components, and reflow (heating and cooling). In a screen-printing apparatus for printing solder paste onto the FPC, a screen mask having provided therein openings to correspond to mounting locations of electronic components is placed on the FPC so as to be abutted against the FPC. Then, solder paste is applied onto the screen mask. The solder paste on the screen mask is pushed (squeegeed) by a squeegee against the FPC and filled in the openings of the screen mask, whereby the solder paste is printed on the FPC being abutted against the underside of the screen mask. After printing, the screen mask is gently lifted and removed from the FPC. In this manner, the solder paste is printed on the mounting locations of the electronic components of the FPC in desired patterns.
In this process, however, if an adhesive layer is formed to be larger than an FPC to be held, when a screen mask is placed on the FPC, a portion of the screen mask which is not being abutted against the FPC may be attached to the adhesive layer, causing the following problems. Specifically, a portion of the screen mask which is being abutted against the FPC is only being placed on the FPC, and thus can be removed from the FPC by lifting the screen mask without any resistance. On the other hand, a portion of the screen mask which is being attached to the adhesive layer is being securely adhered to the adhesive layer, and therefore when the screen mask is lifted, the screen mask is pulled by the adhesive layer and then deformed. Here, if the screen mask is further lifted, the screen mask will be eventually removed from the adhesive layer, however, the screen mask may be further deformed and swayed during the further lifting.
Due to the deformation and swaying of the screen mask, when the screen mask is removed from the FPC, the solder paste being filled in the printing openings may be damaged; as a result, the solder paste cannot be formed on the FPC in predetermined patterns. Accordingly, electronic components cannot be properly formed on circuits of the FPC. Further, under normal conditions of use in which the screen mask is gently lifted, the screen mask has durability and thus can be used repeatedly. However, deformation of the screen mask caused by the adhesive layer pulling the screen mask may cause deformation of the shape of the openings of the screen mask, which in turn makes it impossible to repeatedly use the screen mask. Thus, attachment of the screen mask to the adhesive layer is very inconvenient as it may cause degradation of mounting quality of electronic components and a reduction in yield.
To avoid the aforementioned problems, the screen mask needs to be prevented from attaching to an adhesive layer. For example, an adhesive layer may be provided only in a location of a pallet where an FPC is to be held. Specifically, an adhesive layer may be provided in a region of a base plate which is smaller than the outer shape of the FPC. In this method, in the case where a plurality of FPCs are held onto one pallet, independently-formed adhesive layer regions are provided to be greater in number than FPCs to be held, and to be spaced apart from each other. In this case, a depression (i.e., a height difference between the surface of the adhesive layer and the surface of the base plate) is generated between an adhesive layer region and a non-adhesive layer region on the base plate.
When performing printing on a plurality of FPCs being held onto the above-described pallet, the screen mask does not attach to the adhesive layers. However, the screen mask may be deformed at a depression between adhesive layers or at the outer edge of an adhesive layer. Specifically, when solder paste being applied onto the screen mask is pressed with a squeegee, the screen mask is also pressed on the base plate side through the solder paste, whereby the screen mask is greatly deformed at a depression between adhesive layers.
When the squeegee is moving across adhesive layer regions, the screen mask is being supported by adhesive layers, and thus deformation of the screen mask can be neglected. However, when the squeegee gets to a depression from an adhesive layer region, the screen mask loses its support at the outer edge of the adhesive layer region, whereby the screen mask sinks on the base plate side along with the squeegee and is then greatly deformed. While the squeegee is moving up to a depression, the screen mask may be deformed in conjunction with the movement of the squeegee. Thus, at the point when the squeegee has reached over the subsequent adhesive layer region, deformation of the screen mask has been accumulated at the outer edge of the subsequent adhesive layer region, and in the worst case the screen mask may be caught between the squeegee and the outer edge, thereby damaging the screen mask.
In addition to the damage to the screen mask, the printing operation of the squeegee may become unstable, depending on the depth of a depression (i.e., the thickness of an adhesive layer) and/or the size of a depression (i.e., the spacing distance between adhesive layers). Specifically, in the case where FPCs are held to adhesive layers which are provided only in the direction perpendicular to the moving direction of the squeegee, the squeegee and the screen mask are greatly sunk at the aforementioned depression, whereby the squeegee is tilted and the printing operation becomes unstable.
If the tackiness of an adhesive layer is low, an FPC cannot be held by adhesion due to an air pressure caused by hot air circulation during reflow. On the other hand, if the tackiness of an adhesive layer is high, especially in the case where the contact area between an adhesive material and an FPC is large, when peeling the FPC, a large force may be applied to the FPC; therefore, peeling cannot be performed properly, causing the same problems as the aforementioned problems caused when the screen mask is adhered to an adhesive layer.